When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body's circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via an intravascular leadwire or catheter (referred to as a “lead”) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as “capturing” the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly.
Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn't allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering a high energy electrical stimulus that is sometimes referred to as a defibrillation countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other implantable or external systems or devices for diagnosing or treating cardiac arrhythmias.
One problem faced by cardiac rhythm management systems is the treatment of heart failure (also referred to as “HF”). Heart failure, which can result from long-term hypertension, is a condition in which the muscle in the walls of at least one of the right and left sides of the heart deteriorates. By way of example, suppose the muscle in the walls of the left side of the heart deteriorates. As a result, the left atrium and left ventricle become enlarged, and the heart muscle displays less contractility. This decreases cardiac output of blood through the circulatory system which, in turn, may result in an increased heart rate and less resting time between heartbeats. The heart consumes more energy and oxygen, and its condition typically worsens over a period of time.
A deterioration of the heart's conduction system often accompanies heart failure. Normally, intrinsic signals of the heart's conduction system originate in the sinoatrial (SA) node in the upper right atrium. These signals travel through and depolarize the atrial heart tissue to trigger the contraction of the right and left atria. The intrinsic atrial heart signals are received by the atrioventricular (AV) node which, in turn, triggers a subsequent ventricular intrinsic heart signal that travels through the specialized conduction system (Purkinje Fibers) and depolarizes the ventricular heart tissue such that resulting contractions of the right and left ventricles are triggered substantially simultaneously.
In the above example, when the left side of the heart has become enlarged due to heart failure and the conduction system in the left ventricle is blocked, the ventricular intrinsic heart signals may travel through and depolarize the left side of the heart more slowly than in the right side of the heart. This condition is referred to as left bundle branch block (LBBB). As a result, the left and right ventricles do not contract simultaneously, but rather, the left ventricle contracts after the right ventricle. This reduces the pumping efficiency of the heart. Moreover, in the case of LBBB, for example, different regions within the left ventricle may not contract together in a coordinated fashion.
Heart failure can be treated by biventricular (or left-ventricular) coordination therapy that provides pacing pulses to both right and left ventricles. See, e.g., Mower U.S. Pat. No. 4,928,688. Heart failure may also result in an overly long atrioventricular (AV) delay between atrial and ventricular contractions, again reducing the pumping efficiency of the heart. Providing heart failure patients with improved pacing and coordination therapies for improving AV-delay, coordinating ventricular contractions, or otherwise increasing heart pumping efficiency continues to be an area in which improved techniques and therapy protocols are needed.